Different systems in the posterior hypothalamic nucleus of rats control theta frequency and trigger movement.

Reduced frequency of theta activity is thought to compromise hippocampal function and so behavioural inhibition. The anxiolytic benzodiazepine chlordiazepoxide (CDP) reduces theta frequency when injected into the medial supramammillary nucleus (mSuM), posterior hypothalamic nucleus (PH) and dorsomedial hypothalamic nucleus (DMH). These hypothalamic effects on theta could underlie at least some behavioural effects of benzodiazepines. We have previously shown that in a fixed interval 60-s schedule (FI60), CDP injected into mSuM reduced both theta frequency and behavioural inhibition. The present experiments test the effect of injections into PH and DMH on theta and hippocampal-sensitive behaviour (FI60 and open field ambulation). Systemic CDP (5mg/kg i.p.) released, but PH/CDP (20microg in 0.5microl vehicle) suppressed FI responding, though they both reduced FI theta frequency. In the open field, both CDP i.p. and PH/CDP reduced ambulation, but only the systemic injection reduced ambulation theta frequency. Taken together with previous research, these results support a role for PH in the control of voluntary behaviour. They imply that this function may be suppressed, independently of theta, by benzodiazepines. An anxiolytic effect of PH/CDP in FI60 may, therefore, have been masked by a concurrent action of CDP on the PH motor system. DMH/CDP did not affect behaviour or theta in either experiment, despite the fact that this nucleus is involved in benzodiazepine mediation of risk assessment and the flight response. This suggests that, like the control of theta frequency by the hypothalamus, the neural mechanisms underlying anxiety are distributed in complex networks.